The x-ray tube is an integral and critical part of a CT scanner and represents a relatively expensive component that is frequently the failure mode of the scanner. Ideally, an x-ray tube for CT scanner application must have long tube life under continuous high mA scans and high patient throughput.
As is well known, x-rays are generated in vacuum tubes that comprise an anode and a cathode generally referred to as an electron gun which in turn includes a heatable tungsten filament connected to a high voltage source adapted for emitting a high energy beam of accelerated electrons. The anode is in the form of a metal target displaced a short distance from the cathode. When an accelerated electron beam strikes the metal target on the anode, x-rays are generated within the region of the beam's focus. The impact, through a relatively inefficient process, generates x-rays also known as Bremsstrahlung or breaking radiation. Since only about one percent of the total energy of the accelerated electrons is converted to electromagnetic radiation, a large amount of thermal energy is created at the focal region of the target.
In conventional, fixed anode x-ray tubes the debilitating effect of this resultant head effect is minimized by providing the anode with a through flow of cooling fluid to help dissipate the heat. Nonetheless, the generation of considerable heat at a fixed focal spot creates gross limitations on the energy output capacity of the tube as well as on its limits of continuous operability.
A significant improvement was achieved by the rotating anode x-ray tube which expanded the focal spot on the target from a point to a circle. At first, such rotating anode tubes relied on radiation for heat dissipation; however, this too, quickly proved to be limiting. Although efforts for providing through flow cooling were suggested, such as for example, by Fetter in U.S. Pat. No. 4,309,637, rotating type tubes created a new set of problems. The evacuated region of the tube must be sealed to maintain the necessary vacuum.
A most significant consideration in the design of such rotating anode x-ray tubes is the method of sealing the evacuated region about the rotary shaft. Yoshimatsu and Kozaki catalogue a variety of techniques for applying vacuum sealing to the anode rotary shaft in "High Brilliance X-Ray Sources", Topics in Applied Physics, Volume 22, X-Ray Optics, edited by H. J. Queisser, Springer Verlag, 1977. A relatively recently devised method utilizes a magnetic vacuum seal. A problem that has prevented the widespread use of this new technology is its inability to withstand high temperatures required in high-temperature bake-out, a common technique for evolving gasses from metal parts to assure a maintainable high vacuum.